linux-kernel - Re: [BUG] Re: Linux 6.4.4

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Date:   Mon, 24 Jul 2023 09:36:02 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     paulmck@...nel.org
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        rcu@...r.kernel.org, Greg KH <gregkh@...uxfoundation.org>
Subject: Re: [BUG] Re: Linux 6.4.4

On Sun, Jul 23, 2023 at 11:35 PM Paul E. McKenney <paulmck@...nel.org> wrote:
>
> On Mon, Jul 24, 2023 at 12:32:57AM +0000, Joel Fernandes wrote:
> > On Sun, Jul 23, 2023 at 10:19:27AM -0700, Paul E. McKenney wrote:
> > > On Sun, Jul 23, 2023 at 10:50:26AM -0400, Joel Fernandes wrote:
> > > >
> > > >
> > > > On 7/22/23 13:27, Paul E. McKenney wrote:
> > > > [..]
> > > > >
> > > > > OK, if this kernel is non-preemptible, you are not running TREE03,
> > > > > correct?
> > > > >
> > > > >> Next plan of action is to get sched_waking stack traces since I have a
> > > > >> very reliable repro of this now.
> > > > >
> > > > > Too much fun!  ;-)
> > > >
> > > > For TREE07 issue, it is actually the schedule_timeout_interruptible(1)
> > > > in stutter_wait() that is beating up the CPU0 for 4 seconds.
> > > >
> > > > This is very similar to the issue I fixed in New year in d52d3a2bf408
> > > > ("torture: Fix hang during kthread shutdown phase")
> > >
> > > Agreed, if there are enough kthreads, and all the kthreads are on a
> > > single CPU, this could consume that CPU.
> > >
> > > > Adding a cond_resched() there also did not help.
> > > >
> > > > I think the issue is the stutter thread fails to move spt forward
> > > > because it does not get CPU time. But spt == 1 should be very brief
> > > > AFAIU. I was wondering if we could set that to RT.
> > >
> > > Or just use a single hrtimer-based wait for each kthread?
> >
> > [Joel]
> > Yes this might be better, but there's still the issue that spt may not be set
> > back to 0 in some future release where the thread gets starved.
>
> But if each thread knows the absolute time at which the current stutter
> period is supposed to end, there should not be any need for the spt
> variable, correct?

Yes.

> > > > But also maybe the following will cure it like it did for the shutdown
> > > > issue, giving the stutter thread just enough CPU time to move spt forward.
> > > >
> > > > Now I am trying the following and will let it run while I go do other
> > > > family related things. ;)
> > >
> > > Good point, if this avoids the problem, that gives a strong indication
> > > that your hypothesis on the root cause is correct.
> >
> > [Joel]
> > And the TREE07 issue is gone with that change!
[...]
> > Let me know what you think, thanks!
>
> If we can make the stutter kthread set an absolute time for the current
> stutter period to end, then we should be able to simplify the code quite
> a bit and get rid of the CPU consumption entirely.  (Give or take the
> possible need for a given thread to check whether it was erroneously
> awakened early.)
>
> But what specifically did you have in mind?

I was thinking of a 2 counter approach storing the absolute time. Use
an alternative counter for different stuttering sessions. But yes,
generally I agree with the absolute time idea. What do you think Paul?

Do we want to just do  the simpler schedule_timeout at HZ / 20 to keep stable
green, and do the absolute-time approach for mainline? That might be better
from a process PoV. But I think stable requires patches to be upstream. Greg?

I will try to send out patches this week to discuss this, thanks,

 - Joel